1. Field of the Invention
The present invention relates generally to the production of high purity deionized water and more specifically to the enhancement of deionized high purity water at the point of use.
2. Description of the Prior Art
The requirement for high purity water has evolved in several industries. The water purity requirements of the semiconductor industry are among the most demanding of any industry. While the yield of semiconductor chips is dependent upon a variety of factors, the average chip yield is directly related to the purity of deionized process water. As the size of the geometry used on the chips decreases, the adverse effects on yield of impurities in the deionized (DI) process water increase. Accordingly, the requirement for high purity water in semiconductor processing is widely recognized.
Although there is not a widely accepted standard for electronic grade process water, some of the proposed American Society for Testing and Materials (ASTM) requirements for electronic grade water type E-1 (point-of-use ultrapure water) are given in Table 1.
TABLE 1 ______________________________________ Description Quantity ______________________________________ Resistivity (minimum, 18 (90% of time) megaohms-centimeters) with 17 minimum. SiO.sub.2 (total, maximum, 5 micrograms per liter) Particle count (particles 2 larger than 1 micron, maximum per milliliter) Microorganisms (per 1 milliliter) Total organic carbon (TOC) 50 (maximum micrograms per liter) ______________________________________
However, despite the recognition of the requirement for high purity water in semiconductor processing, providing high purity water at the point of use is still problematic, as described more completely below. "Point of use," as used herein, means the point at which a water supply line and a water inlet of an apparatus, such as sinks and spin rinser dryers used in semiconductor processing, are connected.
A typical high purity water system 50 is illustrated in FIG. 1. A source of water 10, usually from a city water supply or a water well source, is passed first through a first granular activated carbon bed 11 and then a filtering system 12. Granulated activated carbon bed 11 and filter 12 remove contaminants which would damage reverse osmosis system 13 to which the water flows from filter 12. After the water is treated by reverse osmosis, the water is passed through deionizing beds 14 and then through an ozone contactor 15 into a holding tank 16. Typically, holding tank 16 has at least a capacity of several hundred liters and more likely a capacity of thousands of liters.
Water is pumped from holding tank 16 by a pump 17 and passed through a first deozonation unit 18 and then into a DI water polisher 19. Subsequently, water from polisher 19 is passed through a submicron filter system 20 and water from filter system 20 is provided to one or more points of use 90.sub.1, 90.sub.2, . . . , 90.sub.N. Typically, points of use 90.sub.1, 90.sub.2, . . . , 90.sub.N are a series of outlets on a rack 90 in a semiconductor fabrication processing unit 21.
Water not extracted from the high purity water system at points of use 90.sub.1, 90.sub.2, . . . , 90.sub.N flows through rack 90 into ozone contactor 15. Thus, a closed loop through ozone contactor 15, holding tank 16, pump 17, deozonation unit 18, polisher 19 and DI water submicron filter unit 20 is formed for the high purity DI water.
To remove bacterial and other living components from high purity water system 50, i.e., sterilize the high purity water system, the water is circulated through the closed loop bypassing deozonation unit 18 and polisher 19. Ozone from ozone generator 27 is introduced into the water through ozone contactor 15. The ozone kills living organisms in the water as well as living organisms attached to the walls of the water system. Within a short period after sterilization, living organisms are again found in the water system.
Typically, as shown in FIG. 1, after the high purity deionized water is used at a point of use, the rinse water is collected and pumped by a pump 22 through a second granular activated carbon bed 23. The water from carbon bed 23 is passed through filter 24 and subsequently through an ozone, UV, H.sub.2 and O.sub.2 reactor 25. Water from reactor 25 is passed through a second deozonation unit 26 before being returned for processing by reverse osmosis unit 13.
While system 50, as in FIG. 1, provides water at the point of use having a higher quality than the quality of the original source of water 10, high purity water system 50 is not only a source of contaminants, but also system 50 provides an environment in which bacteria and other organisms flourish. The level of contamination introduced by the system is affected by regions of low flow within system 50.
Thus, the quality of water available at a point of use from a typical high purity water system is limited by the design and operation of the system. Further, as the components in the system age, the water quality at the point of use is likely to degrade. Upgrading the water quality to the level given in Table 1 requires upgrading the entire distribution system illustrated in FIG. 1. Such an upgrade is difficult, expensive and time consuming.
Several different techniques have been investigated for improving the water quality of existing water purification systems, such as system 50 illustrated in FIG. 1. These techniques include alternative methods for sterilizing the loop to kill bacteria, using ultraviolet light to lower the organic carbon contaminants, i.e., reduce the TOC level, and placing submicron filters at the point of use. However, the effectiveness of these techniques is limited by the size, configuration and operation of the high purity DI water system.
For example, to completely sterilize system 50 (FIG. 1) requires halting water usage throughout fabrication site 21 and continuing the sterilization process until holding tank 16, all piping, and other components in the system are sterilized. However, even when this is done, the living organisms, as previously described, return to the original level within a short period of time. Therefore, the production capacity of fabrication site 21 could, in some instances, be limited by the DI process water quality and consequently the time required for sterilization of the complete system.
The high purity process water systems currently available are all directed to providing a uniform quality of water at multiple points of use. Accordingly, these systems do not provide a means for compensating for different requirements in water quality at different points of use, or for providing ultra pure water at selected points of use.